This invention relates generally to heat pumps and more particularly to heat pumps which use beds of solid adsorbent to drive the heat pump loop in response to the heating and cooling of beds.
Heat driven heat pumps which use solid adsorbent beds to adsorb and desorb a refrigerant are known in the art. These solid adsorbent beds exhibit the phenomena of adsorbing and desorbing refrigerant vapor in response to the changes in the temperature of the adsorbent. One common example of such solid adsorbent material is molecular sieves, commonly known as zeolite. Other materials which exhibit this phenomena are silica gel, alumina and activated carbon. Most any liquid which can be vaporized can be used as the refrigerant. Water is commonly used as a refrigerant with zeolite while sulfur dioxide is commonly used as a refrigerant with silica gel.
Because such beds desorb refrigerant vapor when heated and adsorb refrigerant vapor when cooled, they can be used to drive the refrigerant around a heat pump loop to heat or cool a selected space. In the heat pump loop the refrigerant is desorbed from the bed as it is heated to drive refrigerant out of the bed to a condenser to condense the vapor. The condensed refrigerant is then expanded through an expansion valve and passes on to an evaporator where the refrigerant is again vaporized. When the bed is cooled, refrigerant vapor from the condenser is adsorbed into the bed to complete the cycle. Because a bed cannot readily adsorb and desorb refrigerant at the same time, two solid adsorbent beds are typically used with one being heated while the other is cooled. The heating and cooling steps are reversed when the beds are heated and cooled to the desired temperature limits during a cycle.
A number of different arrangements have been proposed for heating and cooling the beds of solid adsorbent. One common technique uses a heat transfer fluid with a heat exchange arrangement between the fluid and each solid adsorbent bed so that heat is exchanged between the heat transfer fluid and the bed as the heat transfer fluid is circulated through the heat transfer arrangement. The heat transfer fluid is also connected to an external cooling heat exchanger to cool the fluid and an external heater to heat the fluid. The heat transfer loop may be operated in two different ways. One way is to circulate part of the heat transfer fluid heated by the heater through the bed to be heated and then directly back to the heater for reheating while circulating another part of the heat transfer fluid cooled by the cooling heat exchanger through the bed to be cooled and then directly back to the cooling heat exchanger. Another way is to circulate the heated heat transfer fluid from the heater through the bed being heated, then through the cooling heat exchanger to finish cooling the heat transfer fluid, then through the bed being cooled, and finally back to the heater to finish heating the heat transfer fluid. Such an arrangement is illustrated in U.S. Pat. No. 4,183,227 issued Jan. 15, 1980 to J. Bouvin et al.
None of these prior art systems suggests any particular design criteria for the bed and heat exchanger or method of operation therefor except that the beds are simply heated or cooled until the entire bed has reached the end temperature limits of the cycle step. Good engineering practice suggest that the average heat transfer rate between the heat transfer fluid and the bed be kept as high as possible. This suggests that heat should be transferred between the fluid and bed at all times while the fluid and bed are in a heat transfer relationship with each other. As a result, temperature gradients lengthwise of the bed are to be avoided. Using this criteria, the heating coefficient of performance (COP) is typically on the order of 1-1.5 while the cooling COP is typically on the order of 0.1-0.5. The system performance based on this operation has not been able to economically compete with mechanical compressor heat pump systems.